SONET/SDH systems have been built and used for a number of years. Although differences exist between SONET and SDH, those differences are mostly in terminology. In most respects, the two standards are the same and virtually all equipment that complies with either the SONET standard or the SDH standard also complies with the other. Therefore, for the purposes of this specification, the SONET standard and the SDH standard shall be considered interchangeable and the acronym/initialism “SONET/SDH” shall be defined as either the Synchronous Optical Network standard or the Synchronous Digital Hierarchy standard, or both.
The basic SONET/SDH signal is defined as a Synchronous Transport Signal level 1 (STS-1). An STS-1 frame is an 810-byte data packet comprising transport overhead (the information required to maintain communication) and payload (the data itself).
SONET/SDH systems come in many different configurations, but frequently form a network of SONET/SDH nodes connected by links in a closed loop, known in the art as a “ring.” In a ring network, there are two paths between any pair of nodes, one transporting communications signals clockwise and the other counterclockwise.
To protect communications signals from link failures, SONET/SDH rings use one of these two paths as the service, or working, connection and bandwidth is reserved along the other path as a backup (known in the art as a “protection channel”). When a break or fault occurs in a link, a message is sent out requesting protection switching around the break or fault to maintain communications. The request message is communicated by means of an automatic protection switching (APS) channel which uses a two-byte field, in which the two bytes are referred to as K1 and K2, located within the transport overhead of a frame. This ability to rapidly respond with automatic protection switching around breaks and/or faults has made SONET/SDH systems very popular.
Traditional SONET/SDH systems use the STS-1 line overhead to communicate bytes K1 and K2, which are used for indicating a requested source node address and a selected adjacent destination node address as two ends of the protection switching path that is used to bypass any breaks or faults in a corresponding ‘working-ring-segment’ between those two nodes. The request message is usually followed by a command to switch the data traffic to a protection switching path that is predetermined as per BellCORE generic requirement, GR-1230, which is hereby incorporated by reference. BellCORE GR-1230 concerns SONET/SDH systems that have bi-directional line switched rings (BLSR) and defines the use of bytes K1 and K2 of STS-1 section/line overhead for identifying a protection switching path. GR-1230 requires that the source node and the destination node of a protection switching path around a break or a fault be identified in bytes K1 and K2 in an APS message in BLSR SONET/SDH systems.
The GR-1230 APS channel format for bytes K1 and K2 is as follows:
K1 Byte:                Bits 1–4: Type of request (e.g., Lock out of Protection, Forced Switch—Span, Forced Switch—Ring, Signal Fail—Span, Signal Fail—Ring, Signal Degrade—Protection, Signal Degrade—Span, Signal Degrade—Ring, Manual Switch—Span, Manual Switch—Ring, etc.)        Bits 5–8: Destination Node ID (Address)        
K2 Byte:                Bits 1–4: Source Node ID (Address)        Bit 5: Indication of architecture (Short Path or Long Path)        Bits 6–8: Mode of operation (Line AIS-L, Line RDI-L, Extra Traffic, Bridged and Switched Status, Bridged Status and Idle).        
Bytes K1 and K2 are used for APS channel signaling between line terminating entities for bi-directional protection switching and for detecting alarm indication signal (AIS-L) and Remote Defect Indication (RDI) signals.
In a BLSR SONET/SDH system, bytes K1 and K2 provide the only APS signaling channel. Bits 5–8 of byte K1 indicate destination node ID and bits 1–4 of byte K2 indicate source node ID. As can be readily appreciated, with only four bits available for specifying a node ID, a typical SONET/SDH system can only support up to sixteen nodes for a ring using standard protocol.
Furthermore, bytes K1 and K2 do not support more complex networks than rings (e.g., rings within rings, virtual rings, etc.). This shortcoming exists because the one APS channel and the sixteen nodes maximum are closely tied to the physical working ring and to the physical nodes of the ring.
Thus, it is desirable to provide a SONET/SDH system comprising multiple APS channels. It is also desirable to have a SONET/SDH system that is not limited to a maximum of sixteen nodes. It yet further desirable to provide a SONET/SDH system participating in multiple rings, whether the rings are real or virtual, with multiple APS channels and having a capability to address more than sixteen nodes per ring.